Showing papers on "Partial discharge published in 1993"

Classification of partial discharges

Abstract: For a long time, classification of partial discharges was performed by eye, by studying discharge patterns at the ellipse of an oscilloscope screen. The introduction of digital processing techniques allowed automation of the recognition procedure. These procedures are reported and applied to a number of actual HV constructions which suffer from partial discharges. The results of these tests show that a quite satisfactory recognition of discharges takes place. >

Computer-aided measurement of partial discharges in HV equipment

Abstract: A previously developed method of partial discharge (PD) recognition is used to evaluate PD in HV devices. Using conventional discharge detection (bandwidth approximately 400 kHz), the PD patterns are studied. By means of unambiguous statistical operators like skewness, kurtosis, number of peaks, etc., quantitative information about the discharge source can be obtained. In the case of an unknown defect, a combination of these operators is compared to the same set of operators for a known configuration. In this way, a statement can be made on the character of a discharge source. The use of this technique in evaluating different discharges in HV cable and in 400 kV SF/sub 6/ insulation confirms the usefulness of the new approach. >

Spark-to-glow discharge transition due to increased surface conductivity on epoxy resin specimens

Abstract: The effects of long exposure of epoxy resin surfaces to partial discharges under AC field stress were examined. The use of electrical pulse measuring techniques in conjunction with light emission measurements using a photomultiplier tube, which allowed simultaneous detection of both spark and glow discharges, permitted the establishment of a definite transition from spark to glow discharges. Spark discharges were found to be responsible for the initial surface modification leading to increased surface conductivity, which favored the transition to a pulseless glow type discharge within the cavity. The latter form of discharge was found to prevail over the major portion of the remainder of the exposure period. >

PD pattern recognition with neural networks using the multilayer perceptron technique

Abstract: The partial discharge (PD) pattern recognition capability of a neural network, employing the multilayer perceptron technique with data input based on five discharge pulse form parameters, is examined. Simple discharge sources, consisting of artificially created cylindrical cavities with metallic and dielectric electrodes, are employed. The PD pattern discrimination capability is tested using cavities of equal depth but with different electrodes, and cavities of varying depths but with similar electrodes. Preliminary test results are positive. >

Detection and location of internal defects in the insulation of power transformers

Abstract: With the development of fast, and low-cost analog-digital converters (ADCs) and new mathematical procedures for waveform description, e.g., time encoded signal processing and recognition (TESPAR), novel approaches to detect and locate internal defects in the insulation of power transformers become feasible if the following requirements are fulfilled: the equivalent circuit of the insulating system, suitable for the fast partial discharge (PD) signals with risetimes of approximately 10 ns, is known; the response of the insulating system to PD signals injected at least at three different sites of each winding (top, center, bottom) is calculated, or measured at the signal tap-off points (i.e., bushings); and the pattern matrix of all calculated or measured signals exists: e.g., encoded and signal processes with TESPAR as a reference. The above mentioned requirements are examined for quasi-simultaneous detection and location of internal defects in power transformers. The method is applied to PD measurements on power transformers. >

Pulse height analysis of partial discharges in air

Abstract: The pulse height distribution (PHD) functions of partial discharges (PDs) generated by AC voltage in air are investigated. The electrode systems used allow the investigation of single-source PDs and/or multiple-source discharges. Because voltage and the electrode configuration alter the PHD functions of even a single PD source, the overall effect of voltage an multiple-source discharges is rather complicated. The results indicate that the PHD functions of the multiple-source discharges are the superposition of the distribution functions of the discharges and the component sources acting alone. Knowledge of the PHD functions of the typical PD sources together with some kind of deconvolution technique could help in interpreting the results of PD measurements performed on more complex, real insulation systems and, perhaps, identify the main sources of PD in those systems. >

Paschen's law for a hollow cathode discharge

Abstract: An expression for the breakdown voltage of a one‐dimensional hollow cathode discharge has been derived. The breakdown condition which corresponds to Paschen’s law contains, in addition to the first Townsend coefficient, and the secondary electron emission coefficient two parameters which characterize the reflecting action of the electric field and the lifetime of the electrons in the discharge. The breakdown voltage for a hollow cathode discharge in helium was calculated and compared to that of a glow discharge operating under similar conditions.

Partial discharge. XVIII. Errors in the location of partial discharges in high voltage solid dielectric cables

Abstract: Errors in partial discharge (PD) location on high voltage dielectric cables caused by both mechanical and electrical phenomena are discussed. The sources of error relevant to PD location include errors in measuring time delays, scatter caused by noise and disturbances, pulse propagation distortion, errors in cable length measurement, and trivial errors in experimental technique. Experiments to determine typical PD location error were carried out using a detector with a 10 MHz bandwidth. Extensive tests were carried out on a 200 m, 15 kV cable with metering marks on the outer sheath, as well as a 590 m, 10 kV cable without metering marks. >

Partial discharge. XVII. The early history of partial discharge research

Abstract: For pt.XVI see ibid., vol.9, no.3, p.21 et seq. (1993). Early investigations into partial discharge (PD) phenomena are reviewed to illustrate how little has changed and how much has, in some respects, been repeated during the forty years from 1940 to 1980. The early days of PD research included studies of PD in gas-filled cavities in oil-impregnated paper insulation, broad studies of PD in high voltage apparatus and dielectric materials, and research into PD in turbine generator insulation systems. The development of PD test techniques, development of test apparatus, and development of PD-related standards are discussed. >

Progress of insulation ageing and diagnostics of high voltage rotating machine windings in Japan

Abstract: Recent research into using offline and online diagnostics to study ageing of generator insulation is discussed. Preventive maintenance based on offline diagnostic methods and adequate updating of windings are reviewed. A few examples of online partial discharge measurements and sensor designs of and concepts under development by several manufacturers are presented. >

Quantitative measurement of light emission during the early stages of electrical breakdown in epoxy and unsaturated polyester resins

Abstract: Quantitative light emission studies of the initiation and early growth stages of electrical treeing in synthetic resins have been undertaken to gain insight into the underlying physical mechanisms responsible for these processes. Mains-synchronous photon detection techniques coupled with an ultra-sensitive photomultiplier and large-area light collection optics were used to measure the low-level light emission from pin-plane CT200 epoxy and polyester specimens subjected to 50 Hz AC step ramp electrical stress. Three types of light emission are observed consistently and correspond to (i) electroluminescence related to charge injection (Fowler-Nordheim or Schottky depending on the local electric field), (ii) microdischarge activity with formation of microchannels and (iii) conventional partial discharge activity during tree growth. No material-dependent threshold voltage/field was found for electroluminescence and hence charge injection in these materials.

In-service evaluation of motor and generator stator windings using partial discharge tests

Abstract: Partial discharges (corona) are a symptom of most types of deterioration of motor and generator stator windings rated 4 kV and above. Experience indicates that partial discharges occur years before failure. This leaves sufficient time to plan corrective maintenance to avoid the in-service failure of the motor or generator. This paper describes a new type of partial discharge test which can be performed by plant personnel during normal operation of the machine, thus enabling all users of large machines to plan maintenance. The key requirement of the new test is to remove all the interference caused by other sparking and discharging sources in a plant. Such noise has lead to false indications of stator winding deterioration in the past. The new test uses either high-voltage capacitive sensors, high-frequency current transformers, or 1000 MHz electromagnetic couplers to detect the discharge signals. These signals are processed on a pulse-by-pulse basis by a specialized electronic instrument to remove noise and the remaining partial discharges are then categorized according to number, magnitude, and phase position. The test has been implemented on over 100 machines, and the results appear promising. >

The effect of the definition used in measuring partial discharge inception voltages

Abstract: At present, several definitions of partial discharge inception voltage (PDIV) have been advanced. To establish the value of each of these definitions, four practical electrical insulating fluids are selected, and their PDIV is determined according to three of these different specifications. The results suggest that the use of PDIV as a quality criterion generally is not warranted. The significance of these findings is discussed. >

The effect of insulator charging on breakdown and conditioning

Abstract: As part of a study on HV design concepts for microwave tubes, a number of different insulator designs have been studied. Analysis of the measured DC current, partial discharge activity and breakdown voltage shows that surface charging of insulators is a key mechanism in the breakdown process and in the conditioning process. Insulator parameters are not only the breakdown voltage, but also the conditioning speed and the sensitivity to gas exposure or charge leakage. In all these respects insulators with a field enhancement at the anode are superior. Field enhancements at the cathode are less harmful if stepped insulator shapes are chosen. Effective conditioning requires at least a limited number of breakdowns. With sufficient conditioning breakdowns, all insulator geometries tested reached a breakdown field exceeding 12 kV/mm. >

Partial discharge detecting device for resin-molded transformer

Abstract: A partial discharge detecting device for a resin-molded transformer having no-load switching tap terminals on a high voltage winding thereof. An amplifier amplifies the output detection signal of a partial discharge detecting sensor connected between the tap terminals, and an electro-optic converter converts the output signal of the amplifier into an optical signal. The optical signal is applied through an optical fiber to an opto-electric converter where it is converted into an electrical signal. The electrical signal is applied to an indicator. The partial discharge detecting sensor, the amplifier circuit, and the electro-optic converter are held at a potential corresponding to a potential of the tap terminals.

Approach to partial discharge development in closely coupled cavities embedded in solid dielectrics by the lumped capacitance model

Abstract: The use of the typical ABC equivalent circuit for one cavity in solid dielectrics for internal partial discharge modelling has been common practice for many years. The introduction of an equivalent circuit for two closely coupled cavities permits the investigation of the interaction between discharges in such cavities in a solid dielectric when it is stressed by high AC voltages. In this model circuit, cavities and solid dielectric parts are represented as lumped capacitances, while the resistance and the inductance of the test voltage source are taken into consideration. By using a version of the Electromagnetic Transients Program (EMTP), the waveforms of the voltage across the two cavities, as well as those of the voltage across the neighbouring parts of the solid dielectric, have been traced, and the transient voltage stressing of these parts has been analysed for repetitive discharges in the two cavities.

Partial discharge pattern classification using multilayer neural networks

Abstract: Partial discharge measurement is an important means of assessing the condition and integrity of insulation systems in high voltage power apparatus. Commercially available partial discharge detectors display them as patterns by an elliptic time base. Over the years, experts have been interpreting and recognising the nature and cause of partial discharges by studying these patterns. A way to automate this process is reported by using the partial discharge patterns as input to a multilayer neural network with two hidden layers. The patterns are complex and can be further complicated by interference. Therefore the recognition process appropriately qualifies as a challenging neural network task. The simulation results, and those obtained when tested with actual patterns, indicate the suitability of neural nets for real world applications in this emerging domain. Some limitations of this method are also mentioned.

PD analysis of rotating AC machines

Abstract: The problem of the interpretation of partial discharge (PD) measurements is approached by means of the shape analysis of PD amplitude distributions. Experimental cumulative PD distributions, detected insulation systems of HV rotating machine stator windings, are plotted on a Weibull graph in order to identify multiple PD sources. Examples of the application of PD distribution shape analysis as a diagnostic tool for AC HV stator insulation systems are presented and discussed. The calibration procedure and the significance of average parameters for degradation inference are reviewed. >

From bubbles to breakdown, or vice-versa

Abstract: A review of the present understanding of the electrical breakdown mechanism in dielectric liquids is presented. Two fundamental mechanisms, positive discharge and negative discharge, are emphasized. The positive discharge represents an electronic process occurring in the liquid. A highly overvolted negative discharge would follow a similar mechanism. However, at voltages nearer to the minimum breakdown voltage the negative discharge involves cavity formation, which then dominates subsequent events. In particular, liquid viscosity becomes an important parameter in the subsequent growth. The positive discharge exhibits negligible viscosity effect. By contrast the form of the negative discharge is dominated by the viscosity coefficient.

Partial discharges in HV machines; initial considerations for a PD specification

Abstract: Partial discharge (PD) specifications exist for all types of electrical equipment except motors and generators. Whereas specifications essentially require that the equipment is discharge-free the stator conductor bar insulation of the motors and generators under consideration is micaceous and has excellent PD resistance. Consequently, the intention in a specification would be to permit a certain level of PD. It is therefore particularly important that the problem of overcoming the attenuation between the PD site and the measurement equipment at the winding terminals be solved. Within the general context of examining the feasibility of a PD specification, the problem of PD signal attenuation is addressed. The approach adopted was to seek a region of low attenuation by examining the whole of the possible range of PD measurement frequencies. The investigations described consist of experiments on a full-size 500 MW endwinding model and on two large high-voltage motor windings. Careful screening to reduce radiative pick-up from the calibration generator simulating a PD source and a direct earthing scheme enabled the essential elements of PD pulse transfer through a winding to be determined. The frequencies investigated ranged from kHz to GHz. Pulse transmission and frequency spectrum measurements were made. It is concluded that the PD pulse transmits essentially as a travelling wave, the high-frequency components being highly attenuated. No significant crosscoupling was apparent through the end-winding region. The windings acted as a low-pass filter, the cut-off frequency being dependent on the winding length, and being below 1 MHz. Consequently, it is concluded that it should be possible to measure PD, using equipment with a passband below the low-frequency cut-off, with low attentuation.

Artificial self-organizing neural network for partial discharge source recognition

Abstract: In this paper the application of Kohonen's self-organizing feature map for partial discharge (pd) source recognition is described. Several HV tests on artificially injured medium voltage cables have been made to study the capabilities in source recognition and injury description. A description of the learning algorithm and results of the discrimination between different kinds of defects are presented.

Method and apparatus for measuring partial discharges in cables

Abstract: The invention relates to a method for detecting partial discharges occurring in a cable, wherein the cable has an earth screen with a helical structure, comprising the following steps of: arranging at least one detection coil round the cable for detecting electromagnetic pulses travelling along the cable; applying a potential difference between the core and the earth shield of the cable; and analyzing pulses generated by possible partial discharges in the at least one detection coil in order to determine the location and quality of the partial discharge having caused the pulse, wherein the detection coil is adapted to process signals with a frequency higher than 100 MHz. It has been found that with coils comprising only such a limited number of turns the signal detected by the coil is a much better representation of the form of the electromagnetic pulse travelling along the cable.

A partial discharge monitor for the measurement of partial discharges in a high voltage plant by the transient earth voltage technique

Abstract: Several proprietary discharge detectors are available that are suitable for measuring the magnitudes of the partial discharges in plants such as high voltage metalclad switchgear. Their main disadvantages are the high capital cost of the test equipment and the complexity of the testing procedure. When performing such tests, the plant must be switched out, connections must be made to the high voltage conductors of the plant under test and high voltages applied from a test set. A 'sanction for test' is therefore required. This overhead may be acceptable when testing primary or grid substations. However, for discharge measurements to be widely applicable it is essential that the tests can be made with the plant live and under load. The Transient Earth Voltage or TEV measurement technique has been developed to fulfil this requirement.

Acoustic monitoring of partial discharges in gas insulated substations using optical sensors

Abstract: A study has been made of the feasibility of applying optical fibre sensing technology to acoustic monitoring of partial discharges within gas insulated substations (GIS). A test rig has been constructed to simulate a section of GIS in which acoustic sources could be placed. This has been used to test the optical systems which showed greatest promise of having the capability of monitoring these partial discharges. Experimental results are presented which show that acousto-optical sensing, using fibredyne interferometry, of partial discharges within GIS is feasible in principle. However, further work is needed to extend the sensitivity range to acoustical amplitudes of about 10−14 m which correspond to signals from the weakest discharges requiring detection.

Partial discharge measurements in dielectric liquids under impulse voltage

Abstract: Current pulses caused by partial discharges (PDs) are measured using light-emitting diodes (LEDs) and fiber optics under impulse voltage conditions. This system has the advantage of having good isolation from the HV power supply and no effect from electromagnetic noise as compared with ordinary systems. It can detect a PD pulse having a half width of 3 ns, and has the capability of measuring PD pulse charges in the range of 1 to 2000 pC, when the optimum DC bias is applied. PD pulses and light emission or streamer formation in dielectric liquids are measured simultaneously by using an image converter camera. >

Digital measurement of partial discharges in full-sized power capacitors

Abstract: Partial discharges measurements in power capacitors have been carried out using a digital technique. The advantages of the use of this digital technique are discussed. The digital equipment design is presented. Test results on industrial all-film power capacitors with high capacitance values are presented together with measurements of the PDIV before and after the endurance tests. >

Time-resolved discharge measurements

Abstract: This paper gives a short introduction in time-resolved discharge measurements and elucidates this detection method with the results obtained from measurements with 1 GHz bandwidth on polyethylene samples under prolonged discharge activity. It is shown that a correlation between the pulse shape and the physics of the discharge process is possible and that the start of severe degradation of the dielectric can be recognized in the shape of the discharge pulses.

Detecting partial discharge using a detection coil and analysis of output signal and noise frequency spectrums

Abstract: In an electric apparatus such as an electric power cable system, a high frequency pulse is induced on a conductive member such as a metal sheath, where partial discharge occurs in an insulation. The high frequency pulse is detected by means of a bypass lead wire, a metal foil electrode, a detecting coil, and the like provided on the electric apparatus without interrupting a live operation of the electric apparatus.

PD-related stresses in the bulk dielectric and their evaluation

Abstract: The application of electromagnetic field theory to the subject of partial discharges shows that discharging in a void generates large field distortions within the bulk dielectric in the proximity of the void Such inherent over-stressing of a dielectric could be the effect which triggers the onset of electrical treeing and other damaging processes, and which subsequently precipitates the breakdown of the insulation If there were a train of partial discharge events per power frequency cycle, then, during each half period, these events would lead to cumulative stress levels within the solid dielectric >